There is a trend today in the automobile industry to operate the various vehicle accessories, such as the power steering pump, oil and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crankshaft. This system is referred to as a "serpentine" drive belt system when both sides of the belt are used to drive pulleys. To provide optimum operating efficiency for these various accessories and for satisfactory belt life, it is necessary that the drive belt be maintained at a predetermined tension. Due to the relatively greater length for the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension on the belt regardless of the amount of belt stretch.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioning constructions have the disadvantage that the bushing must serve as the spring, provide the required damping and also maintain the alignment of the arm and all of these functions are therefore compromised. The spring rate comprise results in belt tensioner variation and the damping compromise results in a lack of motion control. Also, the bushing softness allows the arm to deflect resulting in less alignment control of the arm and pulley.
Numerous other types of belt tensioning devices use coil springs which are either in compression or tension, for applying and maintaining the tensioning force on a belt-engaging idler pulley or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019; 2,893,255; 3,413,866; 3,483,763; 3,631,734; 3,768,324; 3,812,733; 3,924,483; 3,965,768 and 4,108,013. Some of these coil spring-actuated devices use the biasing force of a spring in combination with hydraulic-actuated members for regulating the amount of tensioning force applied to the belt, depending on whether the engine is running or shut off. Examples of these combination spring and hydraulic belt tensioners are shown in U.S. Pat. Nos. 2,051,488; 3,142,193; and 4,077,272.
Other known belt tensioner constructions, such as shown in U.S. Pat. No. 3,924,483, use a torsional spring for pivotally moving one of the vehicle accessories to achieve the desired tensioning force. Other constructions, such as shown in U.S. Pat. Nos. 3,136,170; 3,483,763; 3,834,246; and 4,285,676, use a torsional coil spring for pivotally moving a lever and idler pulley into belt tensioning engagement which provides a relatively simple, economical and compact unit. U.S. Pat. No. 4,473,362 shows still another belt tensioner which uses a torsional coil spring to provide a variable damping force by applying the radial forces exerted by the volutes of the spring against an internal elastomeric bushing.
It is desirable that a belt tensioner be provided with some type of damping means to prevent excessive oscillation from occurring in the spring tensioning member, and which will absorb sudden shocks to prevent a whipping action from occurring in the tensioner and drive belt. This damping action is especially critical when a coil spring is used for applying the belt tensioning force since coil springs are highly susceptible to developing natural oscillating frequencies when the counter force, which is exerted thereon by the belt, fluctuates during acceleration action. Such fluctuations effect the efficiency of the tensioning force applied to the belt by the coil spring.
Various damping devices have been used with belt tensioners to eliminate or reduce this problem of coil spring oscillation. One type of construction uses a hydraulic fluid as the damping means, such as shown in U.S. Pat. Nos. 2,893,255; 3,964,311; and 3,986,407. U.S. Pat No. 3,710,634 shows a belt tensioner which uses an eccentrically mounted mechanical pinion and rack arrangement which is spring biased by a leaf spring for absorbing an excessive amount of shock as opposed to providing a damping action for spring-biased belt tensioning plunger.
It also is highly desirable when developing a belt tensioner intended primarily for use on an automobile to devise a construction which can be produced as inexpensively as possible without sacrificing durability and efficiency, since a savings of only part of a dollar will amount to a sufficient overall savings when considering the millions of vehicles that are produced by the various vehicle manufacturers on which such belt tensioners will be mounted.
A predetermined amount of frictional torque is required to be exerted against the lever arm in order to stabilize the lever arm and idler pulley and keep it from bouncing continuously during fluctuations in belt loading caused by the cyclic acceleration and deceleration of the engine during idle and the like. It is difficult to get the desired amount of damping torque with existing belt tensioner constructions when the damping force is applied to the bearing on which the lever arm is pivotally mounted since the radius from the pivotal centerline to the point of sliding contact is relatively small thereby providing a corresponding relatively small effective arm length at which the bearing friction acts and which results in frictional damping torque. Therefore, to increase frictional torque when using the bearing surface to provide the frictional damping, it has been necessary to go to a material of higher frictional coefficient or to employ a bearing geometry capable of increasing the radial bearing loads or to increase the effective arm length between the bearing friction surface and the pivotal centerline. It has been found that a frictional torque of approximately 25% of the spring torque is sufficient to suppress nearly all of the vibration of the drive belt tensioner.
Most prior tensioner constructions have insufficient frictional damping available from the drive belt force acting on their pivot bearings. They must provide additional and costly means of producing more damping torque. This is often accomplished by utilizing the forces and friction inherent in a torsion spring acting on the pivotal shaft of the arm by adding a bearing to reduce the wear between the spring and the shaft. It is some times accomplished by providing additional friction surfaces or viscous damping units, all of which increase the cost. It is desirable in belt tensioners to dissipate as quickly as possible the heat that is generated within the tensioner by the sliding friction of the lever arm on its pivot bearing, since such heat determines appreciably the life of the bearing.
Therefore, the need has existed for an improved belt tensioner having an improved damping arrangement in which the bearing generates all of the required damping friction in direct relationship to the force applied on the idler pulley by the drive belt instead of having an additional secondary means to generate the damping friction as heretofore achieved by prior belt tensioners.